uninitialized variables can generate illegal glsl

[csynth]

Very helpful project, thank you.

The code
vec4 main() { vec4 bb; return bb; }
minimizes to
vec4 main(){return bb;}

The original is clearly bad code but according to the spec is legal with undefined results.
The minimized version is not legal glsl and will not compile.

I had much more complicated (autogenerated) code where the result was well-defined despite the uninitialized variables.
I don't think it worth posting that here, but could if it helped.

slightly related to #153

[laurentlb]

Thanks for the report, this is clearly a bug. No need to send more code.

(if no one else sends a PR before, I expect to work on it in a couple of months)